In a variety of electronic systems, there can be many analog circuits operating from the same power supply. One example of such a system is a micro-controller unit (MCU), in which the analog circuits are connected between analog power (AVDD) and analog ground (AVSS) terminals. The analog circuits, such as analog-to-digital converters, can be turned ON or OFF and will draw different amounts of current from the power supply. Due to parasitic resistances (or off chip components used to isolate the analog supply from a digital power supply), this change in current can change the effective supply voltage (AVDD−AVSS) seen at the analog blocks in an MCU.
In analog circuits, there are certain bias voltages that are only valid relative to the supply levels. For example, when a CMOS inverter is used as a comparator, its “bias” is the mid-rail voltage, i.e., (AVDD−AVSS)/2. After the bias voltage has been set, if there is any change in AVDD, the bias will no longer be accurate.
When other analog circuits that share the same power supply are turning ON and OFF, changes in the value of AVDD that is supplied to a given analog circuit can occur. As a result of these changes, the bias voltage for that circuit can be inaccurate. The performance of the circuit will depend on the state of the other circuits sharing the power supply. This is particularly undesirable in the case, for example, of an analog-to-digital converter, where the accuracy could change from one conversion to the next, depending on what other analog circuits are turned on or off.